LCDs are commonly formed with a matrix of picture elements or PELs each containing a TFT coupled with liquid crystal material that transmits light in accordance with control signals applied to the TFT. The transmissivity or apparent brightness of the PEL is a function of the polarization of the liquid crystal material which is a function of the magnitude of the drain voltage and of the time during which such voltage is applied in conjunction with a gate signal. After the control signals have been applied, the parasitic capacitance to the TFT temporarily stores a DC value which must be refreshed or recharged to maintain the desired transmissivity over a longer period of time.
With LCD monitors or displays, it is difficult to obtain many precise steps in the transmissivity between a PEL being fully "On" or "Off". By applying different levels or values of drain voltage, a given PEL can transmit different amounts of light and appear to a user to have different brightness levels of "gray scales". Traditional methods for achieving various gray scales include pulse width modulation (PWM) and pulse amplitude modulation (PAM). In PWM, a fixed drain voltage is applied for different periods of time determined by the pulse width. In PAM, different drain voltages are applied for the same amount of time (fixed pulse width). PAM is preferred for high end TFT LCD monitors. Using PAM, the average polarization of a PEL varies with the PEL's light transmission percentage (transmissivity; to produce gray toning. With PAM, pell transmissivity is directly controlled by the analog value of the voltage applied to the PELs drain line while the PELs gate line is activated by a digital gate signal.
Current LCD manufacturing processes do not yield completely uniform or predictable ranges of PEL transmissivities versus applied PEL drain bias voltages. The basic shape of a transmissivity curve is well understood, but the absolute values thereof vary widely within the displays produced in a given manufacturing run. Human visual perception further complicates the situation because such perception "sees" gray scales in a logarithmic manner as opposed to a linear gradation. This means that to increase the number of gray scales by a factor "n" requires increases in contrast ratios of (.sqroot.2).sup.n where the contrast ratio CR is the ratio of the maximum transmissivity to the minimum transmissivity.
For a given manufacturing run, the values of the different drain voltages to be used for PAM can be preset to the same values for all displays, or each display could be individually factory calibrated. The problem with the former method is that there might be wide variations in the looks of the different displays and/or the number of gray scale levels has to be limited in order to produce acceptable levels. The latter method increases the cost of displays and does not take into account the aging of components such that initial settings may not be satisfactory over time.